In the handling, installation and connection of plastics pipes, the pipe surface is easily damaged. In “no-dig” plastics pipe installation techniques, for example, a tunnel is bored in the ground for the pipe and the pipe is then pushed or pulled through the tunnel into an excavated hole where the next pipe joint is to be made. Installation techniques such as pipe-bursting and slip lining can also place extreme stress on the pipe surface.
Other modern pipe laying methods can also subject the pipe to substantial bending and tensile forces, both of which can result in a deterioration of the mechanical strength of the pipe. In addition, the useful life of the pipe may be reduced by diffusible materials in the ground, or by environment conditions, for example, exposure to direct sunlight for long periods.
Of greatest concern, is that modern pipe laying methods can result in the pipe becoming scratched and dirty. This is disadvantageous firstly as the pipe material may be notch sensitive, in which case any scratches may cause greater damage to occur in the pipe during subsequent handling or use. Secondly, dirt and/or oxidation on the pipe surface may prevent successful welding. The main reason for failure of joints using an electrofusion fitting is that the surface of the pipe is dirty or has become oxidised. For this reason, until recently, the pipe ends always have had to be cleaned and scraped before jointing, for example, with a hand or mechanical scraper. In practice, the cleaning and scraping is often uneven (the underside of the pipe in particular may be treated less carefully), and the quality of the end result depends upon the professional skill of the installer.
In recent years there have been proposals to provide the pipe with a non-adherent skin layer which can be removed in order to permit jointing. Pipe constructions of this type are described, for example, in JP3-24392, JP5-263984, EP0474583, EP0604907, GB2323556, GB2300456, and WO93/00212. The entire disclosures of all these patents and applications are incorporated herein by reference for all purposes.
All of these prior art pipe constructions suffer from the disadvantage that modern pipe laying techniques tend to cause wrinkling, rucking, or at least undesired relative movement of the non-adherent skin layer relative to the core when the pipe is pushed through the ground. These proposals have therefore not proved commercially acceptable.
More traditional proposals, wherein a protective skin layer is provided which is strongly adherent to the pipe, do not, of course, overcome the problem of dirt and oxidation on the outer surface, since such skin layers are very difficult to remove without elaborate equipment. The presence of a tightly adherent skin layer can also dramatically lower the impact strength of the plastics pipe.
The first appreciation that the above problems could be solved by using a protective skin layer which is only lightly adherent to the core pipe occurs in GB2297137 and GB2297138, the entire disclosures of which are incorporated herein by reference for all purposes.
In GB2297138, for example, there is provided a plastics pipe which comprises an inner core and an outer protective layer bonded thereto, in which the dimensions of the pipe and the protective layer are such that the ratio of the external diameter of the pipe to the thickness of the protective layer is at least 70, preferably at least 100, and the cohesive strength of the outer protective layer, excluding any lines of weakness, at least at the ends of the pipe, is greater than the strength of the adhesive bond between the outer protective layer and the inner core. According to this specification, by a correct choice of the material of the skin layer and the extrusion conditions, it is possible to provide a level of adhesion which still permits clean removal of the skin layer by peeling, whilst preventing rucking or wrinkling of the skin layer during installation and without substantially adversely affecting the mechanical properties of the pipe.
GB2297137 and GB2297138 do not recommend the use of an adhesive between the skin layer and the core, relying instead on the Van der Waals and/or diffusive bonding between the polymer surfaces.
The composite pipe of UK patents GB2297137 and GB2297138 has been commercially extremely successful, but it has been found that it is difficult to manufacture a composite pipe which, under specific conditions of temperature and loading, has both the required toughness and limited adhesion of the skin layer to the core pipe. Quality control of the base polymer material of the skin layer, and control of the extrusion conditions during manufacture, need to be rigorously maintained if undesirable quantities of scrap are to be avoided This substantially increases both raw material and manufacturing costs.
The above problem has been addressed in various ways. In WO 04/016976 there is provided an outer removable skin layer that comprises at least two layers, a first outer protective layer and a second inner bonding layer. In WO 04/016420 the outer removable skin layer comprises an adhesion modifying additive. In WO 04/016421 the inner core and the outer removable skin layer are chosen such that the Young's modulus of the skin layer is equal to or less than the Young's modulus of the inner core.
Whilst all these methods have been found to provide significant improvements in manufacturing, they add costs and/or complications to the extrusion process and there still remains a need for a simpler and more economic method for the manufacture of a plastics pipe having a removable protective skin layer.